1. Field of the Invention
The present invention relates to closed loop control systems for controlling a robot or other movable apparatus. In particular, the present invention relates to a feedback system for producing feedback signals representing incremental motion along an axis of movement based upon encoder signals.
2. Description of the Prior Art
With increasing labor costs and with the advent of increasingly sophisticated servo controls, digital electronics and computer technology, industrial robots are finding ever-increasing use. An important part of any robot is the electronic control system, which controls movement of the various members of the robot in one or more axes of movement. The control system typically includes electric or hydraulic servo motors for providing movement along various axes, encoders for sensing incremental movement along each of the axes, feedback circuitry for processing the encoder signals, and control circuitry for providing commands to the servo motors based upon both a stored program of movements and the feedback signals from the feedback circuitry.
The encoders and feedback circuitry typically perform two important functions in the operation of the control system. First, the encoder and feedback circuitry are used during a "teaching" mode in which the robot is moved through the desired sequence of motions in the various axes. The encoder signals are processed by the feedback circuitry, which provides feedback signals to the control circuitry to store data representing the motion required in each of the various axes of movement.
In the "playback" mode, the robot is actually repeating what it has previously been taught to do. In this mode, the control circuitry uses the stored data obtained during the teaching mode to generate command signals. Based upon the command signals and the feedback signals generated during the playback mode, the control circuitry controls the servos to cause the robot to duplicate the movements taught during the teaching mode.
In typical robot controllers, the encoder signals take one of several commonly used forms. In sinusoidal feedback, each encoder produces a sine wave and a cosine wave encoder signal representing motion along one axis. The phase relationship between the sine and cosine encoder signals indicates the direction of movement along the axis.
A second similar type of encoder feedback is quadrature feedback, in which the encoder signals are in the form of a pair of squarewave signals which are ninety degrees phase shifted with respect to one another. The phase relationship between the two squarewave encoder signals indicates the direction of movement along the axis.
A third type of encoder feedback is known as pulsed up/down feedback. In this type of feedback, there are two encoder signals ("UP" and "DOWN"), but at any time only one of the two encoder signals contains a pulse indicating each increment of movement along the axis. The UP encoder signal represents one direction of movement along the axis, and the DOWN encoder signal represents the opposite direction of movement along the axis.
In the past, the feedback circuitry needed to process the encoder signals has required a large number of electronic components. It has not been unusual for the feedback circuitry to require a single circuit board for each axis. In the case of robot controllers which are controlling as many as six different axes of movement, the complexity of the feedback circuitry significantly affects the cost, reliability, and size of the robot controller. There is a continuing need for improved feedback circuitry which utilizes significantly fewer components, which is lower in cost, more reliable and more compact than the circuitry used in the prior art, and which can be used with a wide variety of encoder signal formats.